This invention relates to semiconductor transducers of the type employing piezoresistive sensing elements and more particularly to a transducer structure which employs terminal areas which are fabricated by the implantation of ions.
The semiconductor pressure transducer is a well known device used for sensing pressure or force in a variety of applications. A great many transducers employ pizoresistive devices which in general exhibit a change in resistance proportional to the magnitude of an applied force. The prior art depicts a great number of transducer arrangements which are amenable to mass production techniques as the devices are fabricated by conventional integrated circuit operations. In order to provide a final transducer component, one usually secures the semiconductor transducer which includes a deflecting diaphragm to a suitable housing. In this manner, the housing serves as a support for the diaphragm and furthermore enables the mounting of leads associated with the sensing elements mounted on the diaphragm to enable one to monitor a pressure or force in an environment. The prior art, cognizant of such problems, has proposed many alternate ways of mounting the diaphragm assembly to the housing.
A particularly advantageous technique is described in U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS issued on Apr. 4, 1972 to A. D. Kurtz et al. This patent generally discusses the technique of fabricating an integral silicon diaphragm which includes piezoresistive devices as the force responsive elements. The diaphragm is secured to a housing having slotted sidewalls to enable one to direct leads through the sidewalls in order to obtain a minimum of interference from the leads during diaphragm operation. Hence, in such applications, a portion of the diaphragm which readily deflects when a force is applied thereto is designated as the active region, while the portion of the diaphragm that does not deflect readily is designated as the nonactive region. In order to provide a good seal between the diaphragm and the housing, one must be assured that there is an intimate coupling between the periphery of the housing and the diaphragm. The seal between the diaphragm and the housing should be leak-free to prevent deleterious substances which may be present in the environment from attacking or otherwise interfering with the terminals and sensor components. Furthermore, the nature of the seal specifies that the transducer assembly possesses a surface which is continuous as defined by the surfaces of the diaphragm to be coupled to the housing. Hence, the prior art understands that this surface must present a flat bondable surface to avoid any ridges or flaws which will effect the quality of the seal between the transducer and housing.
In particular, a most efficient and desirable way of protecting transducer elements including the sensors as well as the diaphragm is the technique of covering the piezoresistive elements or other surfaces of the diaphragm with a thin layer of silicon dioxide, which layer serves to protect the disk or diaphragm against deleterious agents present in the force transmitting environment. This can be accomplished by many techniques and provides transducers which are adequately protected and extremely suitable. See U.S. Pat. No. 3,753,196 issued on Aug. 14, 1973 to Kurtz et al entitled TRANSDUCERS EMPLOYING INTEGRAL PROTECTIVE COATINGS AND SUPPORTS. This patent shows certain techniques pertinent to the above.
As indicated, there remains the problem of bonding the diaphragm structure to the housing to form a final transducer assembly. In order to accomplish this, one must be assured that the terminals associated with the transducer structure do not interfere with transducer operation. In coating such transducer with layers of silicon dioxide, one may encounter a problem in regard to the rate of growth of silicon dioxide. For instance it is known that silicon dioxide grows faster on highly doped surfaces than on lightly doped surfaces. Hence, in certin of these structures, a step may be formed during the fabrication process in the clamping region or the nonactive region of the diaphragm. Such a step operates to prevent the formation of a proper seal and one may experience difficulties in attempting to bond such a diaphragm to a suitable housing. This problem is depicted and solved by suitable structure shown in U.S. Pat. No. 3,873,956 entitled INTEGRATED TRANSDUCER ASSEMBLIES issued on Mar. 25, 1975 to A. D. Kurtz, et al. Techniques of fabricating such devices to compensate for the step in the fabrication process are shown in U.S. Pat. No. 3,935,634 entitled METHODS OF FABRICATING INTEGRATED TRANSDUCER ASSEMBLIES issued on Feb. 3, 1976 to A. D. Kurtz, et al.
In any event, consistent with such efforts, the prior art has further sought to develop an improved transducer assembly which will provide flat bondable areas to enable hermetic coupling to a suitable housing. An alternate technique is depicted in U.S. Pat. No. 4,202,217 entitled SEMICONDUCTOR TRANSDUCER EMPLOYING FLAT BONDABLE SURFACES WITH BURIED CONTACT AREAS issued on May 13, 1980.
U.S. Pat. No. 4,208,782 issued on July 24, 1980 entitled METHODS OF FABRICATING TRANSDUCERS EMPLOYING FLAT BONDABLE SURFACES WITH BURIED CONTACT AREAS depicts the technique for fabricating the transducers as disclosed in the above noted patent. In such techniques, a semiconductor pressure transducer includes a base member fabricated from a given conductivity of silicon. The base member has a central depression defining an active area. Diffused in a top surface is at least one contact area which is directed from the active region towards the periphery of the base member. A piezoresistive sensor is located on this surface in contact with the contact area and within the active region. A layer of epitaxial material surrounds the active region and has an aperture on the surface which is in communication with the contact area outside the active region. This epitaxial layer is polished at the top surface and a housing is coupled to this region by means of a suitable bond. In conjunction with such techniques, one can employ transducer configurations which are protected from the environment by means of thin layers of silicon dioxide and achieve both the benefits of a protected device together with proper isolation of terminal and contact areas. Such techniques employ additional steps in the fabrication and hence, are relatively difficult to implement.
An alternate approach depicted in the prior art is to provide a stress sensor apparatus which includes a buried sensor pattern which is fabricated on a semiconductor material by means of ion implantation. This technique is depicted in U.S. Pat. No. 4,035,823 entitled STRESS SENSOR APPARATUS issued on July 12, 1977 to J. F. Marshall and assigned to Honeywell, Inc. This patent attempts to characterize transducers which employ silicon dioxide coatings as suffering in regard to operating characteristics. In essence, the patent indicates that the silicon dioxide layer effects the uniformity of the piezoresistors and hence results in difficult problems in providing compensating techniques to such devices. The patent indicates that the layers adversely effect external stresses on the diaphragm, both in regard to temperature and mechanical stresses. In any event, the patent does not correctly characterize the nature of such devices and it is indicated that the silicon dioxide layer, in fact, is an extremely adequate means of protecting the surface of such a device while being extremely stable and, in fact, does not in any way detract from the performance of the transducer.
However, it is still desirable to provide a most optimum transducer configuration, which configuration will enable one to protect the stress sensors and to provide an optimum bonding surface to enable one to secure the transducer and sensor configuration to a suitable housing. It is an object to provide a sensor arrangement, which arrangement is formed on a semiconductor substrate by a conventional integrated circuit technique such as diffusion or epitaxial growth and to couple the sensor configuration to contact terminal areas by means of terminals which are formed beneath the surface of the diaphragm by means of ion implantation. In this manner, one is enabled to provide a flat surface about the periphery of the sensor array to enable one to couple the array at this surface to a suitable housing.